1. Field of the Invention
The present invention relates to a technology for remotely locking a mobile terminal device.
2. Description of the Related Art
Some mobile terminals can be locked, i.e., the mobile terminal can be put in a state so that it can not accept key input by an unauthorized person. The locking operation includes setting a password. Various methods are available for setting the password. The password can be set, for example, by key-input or audio input. However, these methods are useful when the mobile terminal is available before the eyes. In other words, a password can not be set if the mobile terminal is misplaced or lost.
Japanese Patent Application Laid-Open (JP-A) No. 2000-181865 discloses a technique by which the password can be set by sending an e-mail to the mobile terminal. The email can be sent via any ordinary network, i.e., does not require special infrastructure.
However, the technique disclosed in JP-A No. 2000-181865 uses a conventional and simple password (security code) for authenticating the lock operation, and the password could often be acquired by a malicious third party. Thus, it is comparatively easy for the malicious third party to lock the mobile telephone, and difficult for the owner to prove own identity as the owner of the mobile telephone merely by transmitting the correct password via e-mail. This method therefore lacks security.
JP-A Nos. 2001-268649 and 2001-337929 disclose examples of techniques for improving password security. JP-A No. 2001-268649 uses a partial password consisting of a given digit from a password of, for example, sixteen digits, and JP-A No. 2001-337929 varies the password by using a dynamic security code that changes dynamically over time, thereby making sure that the password is not always fixed to a particular one. In the techniques disclosed in JP-A Nos. 2001-268649 and 2001-337929, digit information relating to a fluctuating password, time lapse information, and the like are presented by the target device such as a mobile telephone at a time of actually using and directly operating the target device, a user inputs a fluctuating password selected or calculated-according to the presented information, and the input password is authenticated. However, when the owner does not possess the target device due to misplacing it or the like, there is no way of learning the fluctuating password information, and consequently, these techniques cannot be applied in a remote e-mail method such as that disclosed in JP-A No. 2000-181865.
Therefore, the conventional e-mail method, although has the advantages of simplicity, is insufficient in improving the security of a lock method for identifying the owner of the mobile telephone and preventing its misuse by a third party.